The orientation column, and orientation-selective cortical neurons, have long been appreciated as the most distinctive emergent features of the visual cortex. Experiments of the past few years have disclosed two anatomical phenomena related to orientation columns: (1) the discovery in our laboratory of the alignment of the collection of receptive fields of geniculocortical afferents that arborize within an orientation column, and (2) the discoveries from Singer's and Katz's laboratories of the development of the patchy intrinsic corticocortical connections that appear from the findings of Gilbert and Wiesel to connect columns of similar orientation preference in adult animals. These two phenomena mirror two contrasting views of the mechanism that produces orientation- selective responses in adult animals: the Hubel and Wiesel (1962) model of simple cells, and the specification or refinement of orientation selectivity by local corticocortical connections. These two aspects of afferent and corticocortical organization might be either causes or consequences of the development of orientation selectivity and orientation columns. We propose to find out which if either of these phenomena is responsible for the development of orientation selectivity and the formation of orientation columns by first determining the times in normal development when each of these kinds of organization emerge in relation to the orientation-selectivity of single neurons. We will then interfere with the development of orientation selectivity by changing afferent and cortical activity by a variety of means and will assess effects on the development of afferent receptive- field alignment and the formation of patchy corticocortical connections. The results of these experiments will be essential for a comprehensive understanding of the mechanisms that organize the visual cortex in development. This understanding should yield insight into the mechanisms responsible for amblyopia, a visual disorder that affects up to 2% of children, as well as possible approaches to therapy.